(1) Field of the Invention
The present invention relates to a process for the preparation of fluorine-containing polymer films, in which a fluorine-containing polymer film is formed simultaneously with polymerization of a fluorine-containing monomer. More particularly, the present invention relates to a process suitable for the production of a film of a perfluorocarbon type polymer having a crosslinked structure.
(2) Description of the Prior Art
Since violent generation of heat is caused in polymerization for the production of fluorine-containing polymers, that is, polymers (inclusive of copolymers; the same will apply hereinafter) of vinyl monomers having at least one fluorine atom in the molecule, bulk polymerization is not ordinarily carried out but suspension polymerization or emulsion polymerization using a liquid medium is ordinarily adopted.
Since a fluorine-containing olefin having 2 to 3 carbon atoms, such as tetrafluoroethylene, is gaseous under polymerization conditions, polymerization of this olefin is ordinarily carried out in the liquid phase by bubbling the olefin in a dispersion medium such as water or in the presence of a solvent.
Each of fluorine-containing polymers prepared according to these known processes is ordinarily in the powdery form or granular form. Accordingly, a fluorine-containing polymer film has heretofore been prepared by heating the obtained polymer in the powdery state, or after pelletizing, to form a melt or gel having a mutual adhesiveness and extrusion-molding and/or calendering the melt or gel to form a film.
In this conventional technique, the step of polymerizing a monomer and the step of forming the polymer into a film are indispensable. Moreover, in the case of a perfluorocarbon type polymer, the melting temperature is ordinarily high and large quantities of heat and mechanical energy are necessary for melt molding. Furthermore, since a surface active agent, a suspending agent such as a salt, an emulsifier, a suspension stabilizer and the like are used for polymerization of a fluorine-containing vinyl monomer, these chemicals adhere to the formed polymer and are contained as impurities. These chemicals are discolored under heating at the step of molding the fluorine-containing polymer, and it often happens that undesirable coloration is caused in the obtained fluorine-containing polymer film.
In the case where a fluorine-containing polymer is used in a special field, for example, as a material for the medical treatment, dissolution of impurities, such as those mentioned above, often violates stipulations of the medical standards.
In the case where a melt or gel formed by heating is molded, the starting polymer should be thermoplastic. It is substantially impossible to obtain a film having a dimension stability improved by crosslinking from a thermoplastic polymer.